The invention relates to tires, and more particularly, to a tread for truck tires for long distance highway travel.
Highway travel, because it involves driving long distances in a substantially straight line with relatively few turns, normally causes low wear in truck tires. However, tires used on highways have been found to experience abnormal wear patterns, which typically appears in three different forms. One form, called xe2x80x9crail wear,xe2x80x9d occurs where the edge of a rib wears differently than the main portion of the rib, and may appear as shallow pits or recesses at or near rib edges that eventually propagate into and across the rib. A second form, called xe2x80x9cflat spotting,xe2x80x9d usually occurs across the surface of a rib and results in a flat spot being generated in the normally curved surface of the rib. A third type of abnormal wear results in a depression of a rib surface about the entire circumference of the tire. Among other problems, abnormal wear can also generate noise and vibrations that may be transmitted through the vehicle suspension to the driver.
It is thought that stress concentrated at the rib edge contributes to the onset of abnormal wear. Accordingly, making the rib edges less stiff than the rest of the rib is believed to help alleviate abnormal wear. One approach along these lines has been to form the grooves between ribs with negatively sloped walls, that is, the grooves widen from the tread surface to the groove bottom. A difficulty with this approach is that these grooves are also more likely trap and retain stones, which can work down into the groove and damage the tire casing.
The present invention proposes a solution to abnormal wear in a tread that may be used for new tires or for retread tires.
According to the invention, a tire tread has at least one groove extending circumferentially about the tire, the groove having side walls that are shaped with protrusions and recesses that alternate in the circumferential direction along the wall surfaces. The vertices of the protrusions and recesses are located between the upper surface of the tread and the bottom of the groove, so that respective bases surround the protrusions and recesses. The bases generally align with a reference plane normal to the groove bottom wall that passes through respective upper edges of the groove.
A unique feature of a preferred embodiment is that the side wall protrusions and recesses protrude and recess relative to the reference planes. This feature ensures that a space exists below the rib edges over the entire circumference of the tread to provide flexibility to the rib edges.
The groove side walls in accordance with the invention may also be defined as a contour incorporating relatively staggered waveforms. At the bottom of the groove the side walls follow a first waveform and between the groove bottom and the tread upper surface the side walls follow a second waveform, the first and second waveforms being mutually out of phase or relatively staggered. The second waveform is preferably located at half the groove depth, that is, midway between the tread upper surface and the groove bottom.
By waveform is meant a contour having deviations from a straight line, including curves or angles, or a combination. According to a preferred embodiment, the waveforms are regular geometric forms, including sinusoidal, sawtooth or zigzag, step waves, or others.
Preferably, the first and second waveforms on a side wall are 180xc2x0 out of phase, that is, the protrusions of one waveform are vertically aligned with the recesses of the other waveform. In addition, the amplitude and period of the waveforms can vary relative to one another.
According to a preferred embodiment of the invention, the first and second waveforms are identical waveforms, for example, both being sinusoidal waveforms. Alternatively, the first and second waveforms can be different forms, for example, the first waveform being a sine wave and the second waveform being a zigzag.
According to a preferred embodiment, the waveforms on opposing side walls of a groove are relatively positioned 180xc2x0 out of phase, a bulge on one wall opposite a cavity on the opposing wall, to produce an intermediate space following the waveform pattern. Alternatively, the waveforms on opposing side walls can be positioned at other relative positions.
According to another aspect of the invention, the edges of the groove walls at the upper surface are formed as linear in the circumferential direction.
Alternatively, and in accordance with a particularly advantageous embodiment of the invention, the upper edges of the groove walls can be formed with a third waveform, the third wave form being out of phase with the second waveform.
The invention may also be incorporated in laterally directed grooves formed in the tread.
The tread may include sipes at the lateral edges of the ribs and sipes may also be formed on the upper surfaces of the side wall waveforms.
The invention advantageously provides flexibility to the rib edges, helping to avoid the stress concentrations that initiate abnormal wear, by providing the cavities below the rib edges. In addition, the shape of the groove space helps avoid trapping stones.